Brake failure while engine running

I am hoping that someone who has more wisdom and knowledge than me can help. I just replaced the brake master cylinder on a 1989 Acura Integra due to a fading pedal. After replacing the master cylinder, I bled the brake lines (manual pump vacuum at the wheels on bleeder).

Now, here is where the problems start. With the engine off, the pedal is firm and does not fade. When the engine is turned on, the brakes go immediately to the floor. When I turn the engine off, the brakes seem to return quickly.

What could this be? Maybe still air in the line? Failing booster? Vacuum problem? I am stumped. Any help is greatly appreciated.


did you bench bleed the MC prior to installation?

did you make sure the rod which goes from the MC into the booster is adjusted correctly, MAX travel, to FULLY bleed the MC? if it isn’t adjusted properly there is an unbled pocket of air in the MC which gives this problem you have.

did you bleed the wheels in the correct sequence? usually RR, LR, RF, LF.

when bleeding with a pump, did you ensure that the bleed screw wasn’t sucking air each time you let up on the pump? (this is why i use the soda bottle/clear hose rig, and this also has the benefit of cleaning out the brake fluid too) Just make sure you DONT run the reservior out of fluid, then you have to start again.

With the engine off you are not getting any power assist from the vacuum booster, which is why the pedal feels firm. You still have air in your system somewhere. Cappy208 has it right, you need to start by bleeding the MC. You may also need to cycle the ABS to get all the air out.

I agree with these guys. You still have air in the system.

Sorry to getting back a bit late. Thank you very much to everyone for the help. Sadly, so far nothing has worked.

I did bleed the MC. As a matter of fact, I bled it a couple of times even when it was on the car (access isn’t too bad).

The wheels were also bled. No air sucking back, as the first few times I bled the system, I used a vacuum pump.

Now, today I decided to do everything again just to make sure I didn’t miss anything. I bled the lines using a second person pressing the brakes this time and alternating loosening and tightening the bleed screw. I attached a clear hose to the bleed screw to detect any air in the line. Throughout this process, I didn’t notice any appreciable air, just a few very tiny bubbles. Following the brake bleed (which did not fix the problem), I pulled the hardlines from the master cylinder and bled the master cylinder. After I put everything back together, I tested the brakes. This time the brakes went straight to the floor and can’t be held or pumped to anything better. Since the car was jacked off the ground, I had my friend step on the brakes, only to find that I can, with a small bit of effort, manually spin the rear wheels (L&R) with the brakes to the floor. I checked the bleeding on the front brakes to find that when pumped and the bleed screw open, I am getting only a very, very small amount of fluid through the bleed valve. It takes about 15+ pump and release cycles to get the fluid in the reservoir to drop 0.5 cm when bleeding.

I am at my wits end. It seems that the problem is getting worse and I have flushed 2 large bottles of brake fluid through the system so far. Could my new master cylinder be bad? Is there an alternative way to get air out of the lines in the event I still have air? Help please! I am covered in brake fluid and can’t think of anything else to do.